it seems your firefox use client side decoration which is using gtk3.
you can enable or disable it via customize option https://www.maketecheasier.com/enable-csd-support-firefox-for-linux/
- Nov 11 2018

the set up is as follows
xfce-panel with xfce4-whiskermenu-plugin, xfce4-dockbarx-plugin, orage for clock and launcher button that showing my avatar icon to launch xfce4-session-logout
'm using compacticons by tatoxXL from deviantart
here is the link:
https://tatosxl.deviantart.com/art/Compacticons-479216575
- Jun 21 2018

i'm sorry, but i haven't tried it on cinnamon desktop yet.
i'd be gratefull if you can post the screenshot of your cinnamon desktop with xfcolorizer theme. may i can fix it in the future release.
i'm using papirus icon with edited places icons to match the dekstop color scheme.
- Sep 14 2017